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Distance from camera

Started by gnitsuk Feb 25, 2008 at 6:27 AM 4 replies 600+ views

Original Post

Feb 25, 2008 at 6:27 AM

Can anyone help me with this one? I have a sprite rendered in a 3D scene using billboarding. As I move the camera further from the sprite so it shrinks in apparent size. As I move the camera closer so it grows. By calling (Just before I render the sprite): m_device.Transform.World = Matrix.Scaling(x, y, 1.0f); I want to keep the sprite the same apparent size. I've got this working with a quick rule of thumb for my app, I just call: float x,y; GetScalingFactors(out x, out y, new Vector3(/*Sprite's x,y,z position*/)); m_device.Transform.World = Matrix.Scaling(x, y, 1.0f); Where GetScalingFactors is defined as: private void GetScalingFactors(out float x, out float y,Vector3 position) { x = 0.01f; y = 0.01f; Vector3 pointToEye = m_eye - position; float DistanceToEye = pointToEye.Length(); if (DistanceToEye == 0.0) { DistanceToEye = float.MinValue; } x = DistanceToEye / 600.0f; y = DistanceToEye / 600.0f; } Then I just call: m_sprite.Draw(m_texture, new Vector3(0.0f, 0.0f, 0.0f), new Vector3(1.0f/x, 1.0f/y, -1.0f), Color.White.ToArgb()); But obviously this is not exact. Can anyone point my to a formula that will exactly compensate for the sprite to camera distance so the the sprite maintains the same size on screen? Thanks, Mitch.

Evil Steve

Feb 25, 2008 at 6:39 AM

Just change to an orthographics projection before rendering the sprite (The C++ function is D3DXMatrixOrthoOffCenterLH(), not sure about C#). That way you won't get the sprite changing size as it moves to and from the camera.

gnitsuk

Feb 25, 2008 at 6:52 AM

Thanks for your reply,

Without going into the details, for my application this isn't what I need.

I do need to do it the way I've indicated,

Thanks again,
Mitch.

chronozphere

Feb 25, 2008 at 7:01 AM

I have tried combining ortho and perspective matrices in one scene and it was problematic. I ran into various problems. You have to deal with the z and w components very carefully.

Check one of my old topics on PGD:

http://www.pascalgamedevelopment.com/viewtopic.php?t=4206&highlight=zbuffer

Hope this helps. :)

Buckeye

Feb 25, 2008 at 6:26 PM

First, I'm not familiar with (I think) C# so I'm not sure how everything works in your code.

However, if I understand what you want to do, just decide how high (or wide) you want the object to appear at a particular distance (say 10 units at a distance of 20 units).

If "actual_height" is the actual height of the object when you create it:

float sizeRatio = picked_height/(2*actual_height*picked_distance);

Keep that value around somewhere.

Then, at a new distance:

float scale = sizeRatio*newDistance;

Scale each dimension with the scale number: newX = oldX*scale, etc.

That should get you started.

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